X-Git-Url: http://plrg.eecs.uci.edu/git/?p=c11tester.git;a=blobdiff_plain;f=model.cc;h=e4757bc6f0d0b919417e60ed9018e0bcd0e86301;hp=96ee3de6325156a75f1952df6cbba0415232ed80;hb=f71cd7c31df98cfc814254da887de9a30c53b31e;hpb=85234dee853ecfdc0ccdb44703a1119b806a465b

diff --git a/model.cc b/model.cc
index 96ee3de6..e4757bc6 100644
--- a/model.cc
+++ b/model.cc
@@ -11,6 +11,7 @@
 #include "cyclegraph.h"
 #include "promise.h"
 #include "datarace.h"
+#include "mutex.h"
 
 #define INITIAL_THREAD_ID	0
 
@@ -27,10 +28,11 @@ ModelChecker::ModelChecker(struct model_params params) :
 	action_trace(new action_list_t()),
 	thread_map(new HashTable<int, Thread *, int>()),
 	obj_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
+	lock_waiters_map(new HashTable<const void *, action_list_t, uintptr_t, 4>()),
 	obj_thrd_map(new HashTable<void *, std::vector<action_list_t>, uintptr_t, 4 >()),
 	promises(new std::vector<Promise *>()),
 	futurevalues(new std::vector<struct PendingFutureValue>()),
-	lazy_sync_with_release(new HashTable<void *, std::list<ModelAction *>, uintptr_t, 4>()),
+	pending_acq_rel_seq(new std::vector<ModelAction *>()),
 	thrd_last_action(new std::vector<ModelAction *>(1)),
 	node_stack(new NodeStack()),
 	mo_graph(new CycleGraph()),
@@ -42,8 +44,6 @@ ModelChecker::ModelChecker(struct model_params params) :
 	priv = (struct model_snapshot_members *)calloc(1, sizeof(*priv));
 	/* First thread created will have id INITIAL_THREAD_ID */
 	priv->next_thread_id = INITIAL_THREAD_ID;
-
-	lazy_sync_size = &priv->lazy_sync_size;
 }
 
 /** @brief Destructor */
@@ -55,13 +55,14 @@ ModelChecker::~ModelChecker()
 
 	delete obj_thrd_map;
 	delete obj_map;
+	delete lock_waiters_map;
 	delete action_trace;
 
 	for (unsigned int i = 0; i < promises->size(); i++)
 		delete (*promises)[i];
 	delete promises;
 
-	delete lazy_sync_with_release;
+	delete pending_acq_rel_seq;
 
 	delete thrd_last_action;
 	delete node_stack;
@@ -83,19 +84,19 @@ void ModelChecker::reset_to_initial_state()
 	snapshotObject->backTrackBeforeStep(0);
 }
 
-/** @returns a thread ID for a new Thread */
+/** @return a thread ID for a new Thread */
 thread_id_t ModelChecker::get_next_id()
 {
 	return priv->next_thread_id++;
 }
 
-/** @returns the number of user threads created during this execution */
+/** @return the number of user threads created during this execution */
 int ModelChecker::get_num_threads()
 {
 	return priv->next_thread_id;
 }
 
-/** @returns a sequence number for a new ModelAction */
+/** @return a sequence number for a new ModelAction */
 modelclock_t ModelChecker::get_next_seq_num()
 {
 	return ++priv->used_sequence_numbers;
@@ -118,12 +119,14 @@ Thread * ModelChecker::get_next_thread(ModelAction *curr)
 {
 	thread_id_t tid;
 
-	/* Do not split atomic actions. */
-	if (curr->is_rmwr())
-		return thread_current();
-	/* The THREAD_CREATE action points to the created Thread */
-	else if (curr->get_type() == THREAD_CREATE)
-		return (Thread *)curr->get_location();
+	if (curr!=NULL) {
+		/* Do not split atomic actions. */
+		if (curr->is_rmwr())
+			return thread_current();
+		/* The THREAD_CREATE action points to the created Thread */
+		else if (curr->get_type() == THREAD_CREATE)
+			return (Thread *)curr->get_location();
+	}
 
 	/* Have we completed exploring the preselected path? */
 	if (diverge == NULL)
@@ -195,58 +198,96 @@ bool ModelChecker::next_execution()
 
 ModelAction * ModelChecker::get_last_conflict(ModelAction *act)
 {
-	action_type type = act->get_type();
-
-	switch (type) {
-		case ATOMIC_READ:
-		case ATOMIC_WRITE:
-		case ATOMIC_RMW:
-			break;
-		default:
-			return NULL;
+	switch (act->get_type()) {
+	case ATOMIC_READ:
+	case ATOMIC_WRITE:
+	case ATOMIC_RMW: {
+		/* linear search: from most recent to oldest */
+		action_list_t *list = obj_map->get_safe_ptr(act->get_location());
+		action_list_t::reverse_iterator rit;
+		for (rit = list->rbegin(); rit != list->rend(); rit++) {
+			ModelAction *prev = *rit;
+			if (act->is_synchronizing(prev))
+				return prev;
+		}
+		break;
 	}
-	/* linear search: from most recent to oldest */
-	action_list_t *list = obj_map->get_safe_ptr(act->get_location());
-	action_list_t::reverse_iterator rit;
-	for (rit = list->rbegin(); rit != list->rend(); rit++) {
-		ModelAction *prev = *rit;
-		if (act->is_synchronizing(prev))
-			return prev;
+	case ATOMIC_LOCK:
+	case ATOMIC_TRYLOCK: {
+		/* linear search: from most recent to oldest */
+		action_list_t *list = obj_map->get_safe_ptr(act->get_location());
+		action_list_t::reverse_iterator rit;
+		for (rit = list->rbegin(); rit != list->rend(); rit++) {
+			ModelAction *prev = *rit;
+			if (act->is_conflicting_lock(prev))
+				return prev;
+		}
+		break;
+	}
+	case ATOMIC_UNLOCK: {
+		/* linear search: from most recent to oldest */
+		action_list_t *list = obj_map->get_safe_ptr(act->get_location());
+		action_list_t::reverse_iterator rit;
+		for (rit = list->rbegin(); rit != list->rend(); rit++) {
+			ModelAction *prev = *rit;
+			if (!act->same_thread(prev)&&prev->is_failed_trylock())
+				return prev;
+		}
+		break;
+	}
+	default:
+		break;
 	}
 	return NULL;
 }
 
+/** This method find backtracking points where we should try to
+ * reorder the parameter ModelAction against.
+ *
+ * @param the ModelAction to find backtracking points for.
+ */
+
+
 void ModelChecker::set_backtracking(ModelAction *act)
 {
-	ModelAction *prev;
-	Node *node;
 	Thread *t = get_thread(act);
-
-	prev = get_last_conflict(act);
+	ModelAction * prev = get_last_conflict(act);
 	if (prev == NULL)
 		return;
 
-	node = prev->get_node()->get_parent();
+	Node * node = prev->get_node()->get_parent();
 
-	while (!node->is_enabled(t))
-		t = t->get_parent();
+	int low_tid, high_tid;
+	if (node->is_enabled(t)) {
+		low_tid = id_to_int(act->get_tid());
+		high_tid = low_tid+1;
+	} else {
+		low_tid = 0;
+		high_tid = get_num_threads();
+	}
 
-	/* Check if this has been explored already */
-	if (node->has_been_explored(t->get_id()))
-		return;
+	for(int i = low_tid; i < high_tid; i++) {
+		thread_id_t tid = int_to_id(i);
+		if (!node->is_enabled(tid))
+			continue;
 
-	/* Cache the latest backtracking point */
-	if (!priv->next_backtrack || *prev > *priv->next_backtrack)
-		priv->next_backtrack = prev;
+		/* Check if this has been explored already */
+		if (node->has_been_explored(tid))
+			continue;
 
-	/* If this is a new backtracking point, mark the tree */
-	if (!node->set_backtrack(t->get_id()))
-		return;
-	DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
-			prev->get_tid(), t->get_id());
-	if (DBG_ENABLED()) {
-		prev->print();
-		act->print();
+		/* Cache the latest backtracking point */
+		if (!priv->next_backtrack || *prev > *priv->next_backtrack)
+			priv->next_backtrack = prev;
+
+		/* If this is a new backtracking point, mark the tree */
+		if (!node->set_backtrack(tid))
+			continue;
+		DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
+					prev->get_tid(), t->get_id());
+		if (DBG_ENABLED()) {
+			prev->print();
+			act->print();
+		}
 	}
 }
 
@@ -281,7 +322,7 @@ bool ModelChecker::process_read(ModelAction *curr, bool second_part_of_rmw)
 			bool r_status = false;
 
 			if (!second_part_of_rmw) {
-				check_recency(curr,false);
+				check_recency(curr);
 				r_status = r_modification_order(curr, reads_from);
 			}
 
@@ -308,6 +349,63 @@ bool ModelChecker::process_read(ModelAction *curr, bool second_part_of_rmw)
 	}
 }
 
+/**
+ * Processes a lock, trylock, or unlock model action.  @param curr is
+ * the read model action to process.
+ *
+ * The try lock operation checks whether the lock is taken.  If not,
+ * it falls to the normal lock operation case.  If so, it returns
+ * fail.
+ *
+ * The lock operation has already been checked that it is enabled, so
+ * it just grabs the lock and synchronizes with the previous unlock.
+ *
+ * The unlock operation has to re-enable all of the threads that are
+ * waiting on the lock.
+ */
+void ModelChecker::process_mutex(ModelAction *curr) {
+	std::mutex *mutex = (std::mutex *)curr->get_location();
+	struct std::mutex_state *state = mutex->get_state();
+	switch (curr->get_type()) {
+	case ATOMIC_TRYLOCK: {
+		bool success = !state->islocked;
+		curr->set_try_lock(success);
+		if (!success) {
+			get_thread(curr)->set_return_value(0);
+			break;
+		}
+		get_thread(curr)->set_return_value(1);
+	}
+		//otherwise fall into the lock case
+	case ATOMIC_LOCK: {
+		if (curr->get_cv()->getClock(state->alloc_tid) <= state->alloc_clock) {
+			printf("Lock access before initialization\n");
+			set_assert();
+		}
+		state->islocked = true;
+		ModelAction *unlock = get_last_unlock(curr);
+		//synchronize with the previous unlock statement
+		if (unlock != NULL)
+			curr->synchronize_with(unlock);
+		break;
+	}
+	case ATOMIC_UNLOCK: {
+		//unlock the lock
+		state->islocked = false;
+		//wake up the other threads
+		action_list_t *waiters = lock_waiters_map->get_safe_ptr(curr->get_location());
+		//activate all the waiting threads
+		for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
+			scheduler->add_thread(get_thread((*rit)->get_tid()));
+		}
+		waiters->clear();
+		break;
+	}
+	default:
+		ASSERT(0);
+	}
+}
+
 /**
  * Process a write ModelAction
  * @param curr The ModelAction to process
@@ -319,7 +417,7 @@ bool ModelChecker::process_write(ModelAction *curr)
 	bool updated_promises = resolve_promises(curr);
 
 	if (promises->size() == 0) {
-		for (unsigned int i = 0; i<futurevalues->size(); i++) {
+		for (unsigned int i = 0; i < futurevalues->size(); i++) {
 			struct PendingFutureValue pfv = (*futurevalues)[i];
 			if (pfv.act->get_node()->add_future_value(pfv.value, pfv.expiration) &&
 					(!priv->next_backtrack || *pfv.act > *priv->next_backtrack))
@@ -333,6 +431,78 @@ bool ModelChecker::process_write(ModelAction *curr)
 	return updated_mod_order || updated_promises;
 }
 
+/**
+ * Initialize the current action by performing one or more of the following
+ * actions, as appropriate: merging RMWR and RMWC/RMW actions, stepping forward
+ * in the NodeStack, manipulating backtracking sets, allocating and
+ * initializing clock vectors, and computing the promises to fulfill.
+ *
+ * @param curr The current action, as passed from the user context; may be
+ * freed/invalidated after the execution of this function
+ * @return The current action, as processed by the ModelChecker. Is only the
+ * same as the parameter @a curr if this is a newly-explored action.
+ */
+ModelAction * ModelChecker::initialize_curr_action(ModelAction *curr)
+{
+	ModelAction *newcurr;
+
+	if (curr->is_rmwc() || curr->is_rmw()) {
+		newcurr = process_rmw(curr);
+		delete curr;
+		compute_promises(newcurr);
+		return newcurr;
+	}
+
+	newcurr = node_stack->explore_action(curr, scheduler->get_enabled());
+	if (newcurr) {
+		/* First restore type and order in case of RMW operation */
+		if (curr->is_rmwr())
+			newcurr->copy_typeandorder(curr);
+
+		ASSERT(curr->get_location() == newcurr->get_location());
+		newcurr->copy_from_new(curr);
+
+		/* Discard duplicate ModelAction; use action from NodeStack */
+		delete curr;
+
+		/* If we have diverged, we need to reset the clock vector. */
+		if (diverge == NULL)
+			newcurr->create_cv(get_parent_action(newcurr->get_tid()));
+	} else {
+		newcurr = curr;
+		/*
+		 * Perform one-time actions when pushing new ModelAction onto
+		 * NodeStack
+		 */
+		curr->create_cv(get_parent_action(curr->get_tid()));
+		if (curr->is_write())
+			compute_promises(curr);
+	}
+	return newcurr;
+}
+
+/**
+ * This method checks whether a model action is enabled at the given point.
+ * At this point, it checks whether a lock operation would be successful at this point.
+ * If not, it puts the thread in a waiter list.
+ * @param curr is the ModelAction to check whether it is enabled.
+ * @return a bool that indicates whether the action is enabled.
+ */
+
+bool ModelChecker::check_action_enabled(ModelAction *curr) {
+	if (curr->is_lock()) {
+		std::mutex * lock = (std::mutex *)curr->get_location();
+		struct std::mutex_state * state = lock->get_state();
+		if (state->islocked) {
+			//Stick the action in the appropriate waiting queue
+			lock_waiters_map->get_safe_ptr(curr->get_location())->push_back(curr);
+			return false;
+		}
+	}
+
+	return true;
+}
+
 /**
  * This is the heart of the model checker routine. It performs model-checking
  * actions corresponding to a given "current action." Among other processes, it
@@ -347,44 +517,29 @@ bool ModelChecker::process_write(ModelAction *curr)
  */
 Thread * ModelChecker::check_current_action(ModelAction *curr)
 {
-	bool second_part_of_rmw = false;
-
 	ASSERT(curr);
 
-	if (curr->is_rmwc() || curr->is_rmw()) {
-		ModelAction *tmp = process_rmw(curr);
-		second_part_of_rmw = true;
-		delete curr;
-		curr = tmp;
-		compute_promises(curr);
-	} else {
-		ModelAction *tmp = node_stack->explore_action(curr);
-		if (tmp) {
-			/* Discard duplicate ModelAction; use action from NodeStack */
-			/* First restore type and order in case of RMW operation */
-			if (curr->is_rmwr())
-				tmp->copy_typeandorder(curr);
-
-			/* If we have diverged, we need to reset the clock vector. */
-			if (diverge == NULL)
-				tmp->create_cv(get_parent_action(tmp->get_tid()));
-
-			delete curr;
-			curr = tmp;
-		} else {
-			/*
-			 * Perform one-time actions when pushing new ModelAction onto
-			 * NodeStack
-			 */
-			curr->create_cv(get_parent_action(curr->get_tid()));
-			/* Build may_read_from set */
-			if (curr->is_read())
-				build_reads_from_past(curr);
-			if (curr->is_write())
-				compute_promises(curr);
-		}
+	bool second_part_of_rmw = curr->is_rmwc() || curr->is_rmw();
+
+	if (!check_action_enabled(curr)) {
+		//we'll make the execution look like we chose to run this action
+		//much later...when a lock is actually available to relese
+		get_current_thread()->set_pending(curr);
+		remove_thread(get_current_thread());
+		return get_next_thread(NULL);
 	}
 
+	ModelAction *newcurr = initialize_curr_action(curr);
+
+	/* Add the action to lists before any other model-checking tasks */
+	if (!second_part_of_rmw)
+		add_action_to_lists(newcurr);
+
+	/* Build may_read_from set for newly-created actions */
+	if (curr == newcurr && curr->is_read())
+		build_reads_from_past(curr);
+	curr = newcurr;
+
 	/* Thread specific actions */
 	switch (curr->get_type()) {
 	case THREAD_CREATE: {
@@ -399,6 +554,8 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 		if (!blocking->is_complete()) {
 			blocking->push_wait_list(curr);
 			scheduler->sleep(waiting);
+		} else {
+			do_complete_join(curr);
 		}
 		break;
 	}
@@ -408,6 +565,7 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 			ModelAction *act = th->pop_wait_list();
 			Thread *wake = get_thread(act);
 			scheduler->wake(wake);
+			do_complete_join(act);
 		}
 		th->complete();
 		break;
@@ -420,10 +578,6 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 		break;
 	}
 
-	/* Add current action to lists before work_queue loop */
-	if (!second_part_of_rmw)
-		add_action_to_lists(curr);
-
 	work_queue_t work_queue(1, CheckCurrWorkEntry(curr));
 
 	while (!work_queue.empty()) {
@@ -440,6 +594,9 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 			if (act->is_write() && process_write(act))
 				updated = true;
 
+			if (act->is_mutex_op())
+				process_mutex(act);
+
 			if (updated)
 				work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
 			break;
@@ -447,8 +604,24 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 		case WORK_CHECK_RELEASE_SEQ:
 			resolve_release_sequences(work.location, &work_queue);
 			break;
-		case WORK_CHECK_MO_EDGES:
-			/** @todo Perform follow-up mo_graph checks */
+		case WORK_CHECK_MO_EDGES: {
+			/** @todo Complete verification of work_queue */
+			ModelAction *act = work.action;
+			bool updated = false;
+
+			if (act->is_read()) {
+				if (r_modification_order(act, act->get_reads_from()))
+					updated = true;
+			}
+			if (act->is_write()) {
+				if (w_modification_order(act))
+					updated = true;
+			}
+
+			if (updated)
+				work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
+			break;
+		}
 		default:
 			ASSERT(false);
 			break;
@@ -462,6 +635,19 @@ Thread * ModelChecker::check_current_action(ModelAction *curr)
 	return get_next_thread(curr);
 }
 
+/**
+ * Complete a THREAD_JOIN operation, by synchronizing with the THREAD_FINISH
+ * operation from the Thread it is joining with. Must be called after the
+ * completion of the Thread in question.
+ * @param join The THREAD_JOIN action
+ */
+void ModelChecker::do_complete_join(ModelAction *join)
+{
+	Thread *blocking = (Thread *)join->get_location();
+	ModelAction *act = get_last_action(blocking->get_id());
+	join->synchronize_with(act);
+}
+
 void ModelChecker::check_curr_backtracking(ModelAction * curr) {
 	Node *currnode = curr->get_node();
 	Node *parnode = currnode->get_parent();
@@ -486,25 +672,38 @@ bool ModelChecker::promises_expired() {
 	return false;
 }
 
-/** @returns whether the current partial trace must be a prefix of a
+/** @return whether the current partial trace must be a prefix of a
  * feasible trace. */
 bool ModelChecker::isfeasibleprefix() {
-	return promises->size() == 0 && *lazy_sync_size == 0;
+	return promises->size() == 0 && pending_acq_rel_seq->size() == 0;
 }
 
-/** @returns whether the current partial trace is feasible. */
+/** @return whether the current partial trace is feasible. */
 bool ModelChecker::isfeasible() {
 	return !mo_graph->checkForRMWViolation() && isfeasibleotherthanRMW();
 }
 
-/** @returns whether the current partial trace is feasible other than
+/** @return whether the current partial trace is feasible other than
  * multiple RMW reading from the same store. */
 bool ModelChecker::isfeasibleotherthanRMW() {
+	if (DBG_ENABLED()) {
+		if (mo_graph->checkForCycles())
+			DEBUG("Infeasible: modification order cycles\n");
+		if (failed_promise)
+			DEBUG("Infeasible: failed promise\n");
+		if (too_many_reads)
+			DEBUG("Infeasible: too many reads\n");
+		if (promises_expired())
+			DEBUG("Infeasible: promises expired\n");
+	}
 	return !mo_graph->checkForCycles() && !failed_promise && !too_many_reads && !promises_expired();
 }
 
 /** Returns whether the current completed trace is feasible. */
 bool ModelChecker::isfinalfeasible() {
+	if (DBG_ENABLED() && promises->size() != 0)
+		DEBUG("Infeasible: unrevolved promises\n");
+
 	return isfeasible() && promises->size() == 0;
 }
 
@@ -531,7 +730,7 @@ ModelAction * ModelChecker::process_rmw(ModelAction *act) {
  *
  * If so, we decide that the execution is no longer feasible.
  */
-void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
+void ModelChecker::check_recency(ModelAction *curr) {
 	if (params.maxreads != 0) {
 		if (curr->get_node()->get_read_from_size() <= 1)
 			return;
@@ -552,12 +751,10 @@ void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
 
 		action_list_t::reverse_iterator rit = list->rbegin();
 		/* Skip past curr */
-		if (already_added) {
-			for (; (*rit) != curr; rit++)
-				;
-			/* go past curr now */
-			rit++;
-		}
+		for (; (*rit) != curr; rit++)
+			;
+		/* go past curr now */
+		rit++;
 
 		action_list_t::reverse_iterator ritcopy = rit;
 		//See if we have enough reads from the same value
@@ -618,7 +815,7 @@ void ModelChecker::check_recency(ModelAction *curr, bool already_added) {
 
 /**
  * Updates the mo_graph with the constraints imposed from the current
- * read.  
+ * read.
  *
  * Basic idea is the following: Go through each other thread and find
  * the lastest action that happened before our read.  Two cases:
@@ -654,7 +851,7 @@ bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf
 			 */
 			if (act->happens_before(curr) && act != curr) {
 				if (act->is_write()) {
-					if (rf != act && act != curr) {
+					if (rf != act) {
 						mo_graph->addEdge(act, rf);
 						added = true;
 					}
@@ -665,7 +862,6 @@ bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf
 						added = true;
 					}
 				}
-
 				break;
 			}
 		}
@@ -678,7 +874,7 @@ bool ModelChecker::r_modification_order(ModelAction *curr, const ModelAction *rf
  *  promises.  The basic problem is that actions that occur after the
  *  read curr could not property add items to the modification order
  *  for our read.
- *  
+ *
  *  So for each thread, we find the earliest item that happens after
  *  the read curr.  This is the item we have to fix up with additional
  *  constraints.  If that action is write, we add a MO edge between
@@ -772,24 +968,33 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 		action_list_t::reverse_iterator rit;
 		for (rit = list->rbegin(); rit != list->rend(); rit++) {
 			ModelAction *act = *rit;
+			if (act == curr) {
+				/*
+				 * If RMW, we already have all relevant edges,
+				 * so just skip to next thread.
+				 * If normal write, we need to look at earlier
+				 * actions, so continue processing list.
+				 */
+				if (curr->is_rmw())
+					break;
+				else
+					continue;
+			}
 
 			/*
 			 * Include at most one act per-thread that "happens
-			 * before" curr. Only consider reflexively if curr is
-			 * RMW.
+			 * before" curr
 			 */
-			if (act->happens_before(curr) && (act != curr || curr->is_rmw())) {
+			if (act->happens_before(curr)) {
 				/*
 				 * Note: if act is RMW, just add edge:
 				 *   act --mo--> curr
 				 * The following edge should be handled elsewhere:
 				 *   readfrom(act) --mo--> act
 				 */
-				if (act->is_write()) {
-					//RMW shouldn't have an edge to themselves
-					if (act!=curr)
-						mo_graph->addEdge(act, curr);
-				} else if (act->is_read() && act->get_reads_from() != NULL)
+				if (act->is_write())
+					mo_graph->addEdge(act, curr);
+				else if (act->is_read() && act->get_reads_from() != NULL)
 					mo_graph->addEdge(act->get_reads_from(), curr);
 				added = true;
 				break;
@@ -805,7 +1010,7 @@ bool ModelChecker::w_modification_order(ModelAction *curr)
 				 */
 				if (thin_air_constraint_may_allow(curr, act)) {
 					if (isfeasible() ||
-							(curr->is_rmw() && act->is_rmw() && curr->get_reads_from()==act->get_reads_from() && isfeasibleotherthanRMW())) {
+							(curr->is_rmw() && act->is_rmw() && curr->get_reads_from() == act->get_reads_from() && isfeasibleotherthanRMW())) {
 						struct PendingFutureValue pfv = {curr->get_value(),curr->get_seq_number()+params.maxfuturedelay,act};
 						futurevalues->push_back(pfv);
 					}
@@ -829,7 +1034,7 @@ bool ModelChecker::thin_air_constraint_may_allow(const ModelAction * writer, con
 		return true;
 
 	for (const ModelAction *search = writer->get_reads_from(); search != NULL; search = search->get_reads_from()) {
-		if (search==reader)
+		if (search == reader)
 			return false;
 		if (search->get_tid() == reader->get_tid() &&
 				search->happens_before(reader))
@@ -860,8 +1065,7 @@ bool ModelChecker::thin_air_constraint_may_allow(const ModelAction * writer, con
  * @return true, if the ModelChecker is certain that release_heads is complete;
  * false otherwise
  */
-bool ModelChecker::release_seq_head(const ModelAction *rf,
-                std::vector< const ModelAction *, MyAlloc<const ModelAction *> > *release_heads) const
+bool ModelChecker::release_seq_head(const ModelAction *rf, rel_heads_list_t *release_heads) const
 {
 	if (!rf) {
 		/* read from future: need to settle this later */
@@ -917,10 +1121,12 @@ bool ModelChecker::release_seq_head(const ModelAction *rf,
 		 * the release seq? */
 		bool future_ordered = false;
 
+		ModelAction *last = get_last_action(int_to_id(i));
+		if (last && rf->happens_before(last))
+			future_ordered = true;
+
 		for (rit = list->rbegin(); rit != list->rend(); rit++) {
 			const ModelAction *act = *rit;
-			if (!act->is_write())
-				continue;
 			/* Reach synchronization -> this thread is complete */
 			if (act->happens_before(release))
 				break;
@@ -929,6 +1135,10 @@ bool ModelChecker::release_seq_head(const ModelAction *rf,
 				continue;
 			}
 
+			/* Only writes can break release sequences */
+			if (!act->is_write())
+				continue;
+
 			/* Check modification order */
 			if (mo_graph->checkReachable(rf, act)) {
 				/* rf --mo--> act */
@@ -966,18 +1176,14 @@ bool ModelChecker::release_seq_head(const ModelAction *rf,
  * with the head(s) of the release sequence(s), if they exists with certainty.
  * @see ModelChecker::release_seq_head
  */
-void ModelChecker::get_release_seq_heads(ModelAction *act,
-                std::vector< const ModelAction *, MyAlloc<const ModelAction *> > *release_heads)
+void ModelChecker::get_release_seq_heads(ModelAction *act, rel_heads_list_t *release_heads)
 {
 	const ModelAction *rf = act->get_reads_from();
 	bool complete;
 	complete = release_seq_head(rf, release_heads);
 	if (!complete) {
 		/* add act to 'lazy checking' list */
-		std::list<ModelAction *> *list;
-		list = lazy_sync_with_release->get_safe_ptr(act->get_location());
-		list->push_back(act);
-		(*lazy_sync_size)++;
+		pending_acq_rel_seq->push_back(act);
 	}
 }
 
@@ -988,7 +1194,7 @@ void ModelChecker::get_release_seq_heads(ModelAction *act,
  * modification order information is present at the time an action occurs.
  *
  * @param location The location/object that should be checked for release
- * sequence resolutions
+ * sequence resolutions. A NULL value means to check all locations.
  * @param work_queue The work queue to which to add work items as they are
  * generated
  * @return True if any updates occurred (new synchronization, new mo_graph
@@ -996,17 +1202,19 @@ void ModelChecker::get_release_seq_heads(ModelAction *act,
  */
 bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_queue)
 {
-	std::list<ModelAction *> *list;
-	list = lazy_sync_with_release->getptr(location);
-	if (!list)
-		return false;
-
 	bool updated = false;
-	std::list<ModelAction *>::iterator it = list->begin();
-	while (it != list->end()) {
+	std::vector<ModelAction *>::iterator it = pending_acq_rel_seq->begin();
+	while (it != pending_acq_rel_seq->end()) {
 		ModelAction *act = *it;
+
+		/* Only resolve sequences on the given location, if provided */
+		if (location && act->get_location() != location) {
+			it++;
+			continue;
+		}
+
 		const ModelAction *rf = act->get_reads_from();
-		std::vector< const ModelAction *, MyAlloc<const ModelAction *> > release_heads;
+		rel_heads_list_t release_heads;
 		bool complete;
 		complete = release_seq_head(rf, &release_heads);
 		for (unsigned int i = 0; i < release_heads.size(); i++) {
@@ -1022,7 +1230,7 @@ bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_
 
 			/* propagate synchronization to later actions */
 			action_list_t::reverse_iterator it = action_trace->rbegin();
-			while ((*it) != act) {
+			for (; (*it) != act; it++) {
 				ModelAction *propagate = *it;
 				if (act->happens_before(propagate)) {
 					propagate->synchronize_with(act);
@@ -1031,10 +1239,9 @@ bool ModelChecker::resolve_release_sequences(void *location, work_queue_t *work_
 				}
 			}
 		}
-		if (complete) {
-			it = list->erase(it);
-			(*lazy_sync_size)--;
-		} else
+		if (complete)
+			it = pending_acq_rel_seq->erase(it);
+		else
 			it++;
 	}
 
@@ -1070,12 +1277,18 @@ void ModelChecker::add_action_to_lists(ModelAction *act)
 	(*thrd_last_action)[tid] = act;
 }
 
-ModelAction * ModelChecker::get_last_action(thread_id_t tid)
+/**
+ * @brief Get the last action performed by a particular Thread
+ * @param tid The thread ID of the Thread in question
+ * @return The last action in the thread
+ */
+ModelAction * ModelChecker::get_last_action(thread_id_t tid) const
 {
-	int nthreads = get_num_threads();
-	if ((int)thrd_last_action->size() < nthreads)
-		thrd_last_action->resize(nthreads);
-	return (*thrd_last_action)[id_to_int(tid)];
+	int threadid = id_to_int(tid);
+	if (threadid < (int)thrd_last_action->size())
+		return (*thrd_last_action)[id_to_int(tid)];
+	else
+		return NULL;
 }
 
 /**
@@ -1086,7 +1299,7 @@ ModelAction * ModelChecker::get_last_action(thread_id_t tid)
  * check
  * @return The last seq_cst write
  */
-ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr)
+ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr) const
 {
 	void *location = curr->get_location();
 	action_list_t *list = obj_map->get_safe_ptr(location);
@@ -1098,6 +1311,26 @@ ModelAction * ModelChecker::get_last_seq_cst(ModelAction *curr)
 	return NULL;
 }
 
+/**
+ * Gets the last unlock operation performed on a particular mutex (i.e., memory
+ * location). This function identifies the mutex according to the current
+ * action, which is presumed to perform on the same mutex.
+ * @param curr The current ModelAction; also denotes the object location to
+ * check
+ * @return The last unlock operation
+ */
+ModelAction * ModelChecker::get_last_unlock(ModelAction *curr) const
+{
+	void *location = curr->get_location();
+	action_list_t *list = obj_map->get_safe_ptr(location);
+	/* Find: max({i in dom(S) | isUnlock(t_i) && samevar(t_i, t)}) */
+	action_list_t::reverse_iterator rit;
+	for (rit = list->rbegin(); rit != list->rend(); rit++)
+		if ((*rit)->is_unlock())
+			return *rit;
+	return NULL;
+}
+
 ModelAction * ModelChecker::get_parent_action(thread_id_t tid)
 {
 	ModelAction *parent = get_last_action(tid);
@@ -1223,11 +1456,11 @@ void ModelChecker::build_reads_from_past(ModelAction *curr)
 			ModelAction *act = *rit;
 
 			/* Only consider 'write' actions */
-			if (!act->is_write())
+			if (!act->is_write() || act == curr)
 				continue;
 
 			/* Don't consider more than one seq_cst write if we are a seq_cst read. */
-			if (!curr->is_seqcst()|| (!act->is_seqcst() && (last_seq_cst==NULL||!act->happens_before(last_seq_cst))) || act == last_seq_cst) {
+			if (!curr->is_seqcst() || (!act->is_seqcst() && (last_seq_cst == NULL || !act->happens_before(last_seq_cst))) || act == last_seq_cst) {
 				DEBUG("Adding action to may_read_from:\n");
 				if (DBG_ENABLED()) {
 					act->print();
@@ -1283,7 +1516,7 @@ void ModelChecker::print_summary()
 #if SUPPORT_MOD_ORDER_DUMP
 	scheduler->print();
 	char buffername[100];
-	sprintf(buffername, "exec%u",num_executions);
+	sprintf(buffername, "exec%04u", num_executions);
 	mo_graph->dumpGraphToFile(buffername);
 #endif
 
@@ -1304,6 +1537,11 @@ void ModelChecker::add_thread(Thread *t)
 	scheduler->add_thread(t);
 }
 
+/**
+ * Removes a thread from the scheduler. 
+ * @param the thread to remove.
+ */
+
 void ModelChecker::remove_thread(Thread *t)
 {
 	scheduler->remove_thread(t);
@@ -1331,25 +1569,23 @@ int ModelChecker::switch_to_master(ModelAction *act)
  * @return Returns true (success) if a step was taken and false otherwise.
  */
 bool ModelChecker::take_step() {
-	Thread *curr, *next;
-
 	if (has_asserted())
 		return false;
 
-	curr = thread_current();
+	Thread * curr = thread_current();
 	if (curr) {
 		if (curr->get_state() == THREAD_READY) {
 			ASSERT(priv->current_action);
 
 			priv->nextThread = check_current_action(priv->current_action);
 			priv->current_action = NULL;
-			if (!curr->is_blocked() && !curr->is_complete())
-				scheduler->add_thread(curr);
+			if (curr->is_blocked() || curr->is_complete())
+				scheduler->remove_thread(curr);
 		} else {
 			ASSERT(false);
 		}
 	}
-	next = scheduler->next_thread(priv->nextThread);
+	Thread * next = scheduler->next_thread(priv->nextThread);
 
 	/* Infeasible -> don't take any more steps */
 	if (!isfeasible())
@@ -1362,6 +1598,15 @@ bool ModelChecker::take_step() {
 	/* next == NULL -> don't take any more steps */
 	if (!next)
 		return false;
+
+	if ( next->get_pending() != NULL ) {
+		//restart a pending action
+		set_current_action(next->get_pending());
+		next->set_pending(NULL);
+		next->set_state(THREAD_READY);
+		return true;
+	}
+
 	/* Return false only if swap fails with an error */
 	return (Thread::swap(&system_context, next) == 0);
 }